Preparation of a novel polyhydroxy silica stationary phase and application in the separation of alginate oligosaccharides

doi:10.3724/SP.J.1123.2025.03006

Abstract

Abstract:

Hyperbranched polyglycerols （HPG） represent an emerging class of dendritic polyethers characterized by a highly branched， three-dimensional architecture and a multitude of terminal hydroxyl groups. This distinctive structural configuration confers exceptional hydrophilicity， biocompatibility， and a high density of modifiable surface functionalities， thus establishing HPG as highly promising materials for the development of advanced chromatographic stationary phases. Their application is particularly relevant for the separation of polar and hydrophilic analytes， which has long posed a significant challenge in conventional reversed-phase liquid chromatography. The integration of such hyperbranched polymers with robust inorganic substrates， such as silica gel， has recently gained traction as a sophisticated materials strategy. This approach synergistically combines the superior mechanical strength and pressure resistance of the inorganic matrix with the rich surface chemistry and tunable hydrophilicity of the polymer， thereby addressing critical limitations of traditional stationary phases in hydrophilic interaction liquid chromatography （HILIC） applications.In the present study， a novel HILIC stationary phase， designated HPG-Sil 3， was synthesized through the in-situ ring-opening polymerization of glycidol monomers from the surface of aminopropyl-functionalized silica microparticles. The synthesis commenced with a meticulous silanization step to graft （3-aminopropyl）triethoxysilane （APTES） onto the silica surface， thereby introducing a uniform layer of primary amine groups. These amine functionalities served as initiation sites for the subsequent grafting-from polymerization， ensuring the formation of a covalently anchored， robust HPG layer. This covalent immobilization strategy is critical for mitigating stationary phase degradation and polymer leaching under prolonged chromatographic use， thereby guaranteeing long-term operational stability. The successful fabrication of the HPG-Sil 3 material and its physicochemical properties were thoroughly characterized using a suite of analytical techniques. Elemental analysis indicated a substantial increase in carbon and hydrogen content post-modification， providing quantitative evidence of organic polymer grafting. Fourier-transform infrared （FTIR） spectroscopy further corroborated this result， revealing signature absorption bands associated with the stretching vibrations of O-H and C-O-C ether linkages， which are characteristic of the HPG polyether structure. Thermogravimetric analysis （TGA） demonstrated the material’s excellent thermal resilience， with the onset of HPG decomposition occurring above 200 ℃， a temperature window far exceeding the operational range of typical HILIC analyses. Textural properties， evaluated via nitrogen physisorption， showed a predictable decrease in specific surface area and pore volume relative to the unmodified silica substrate. This reduction is attributed to the partial filling of the mesoporous silica network by the grafted HPG chains. Importantly， the material retained a sufficiently open porous structure to facilitate efficient mass transfer of analytes during chromatographic runs. To systematically investigate the chromatographic behavior and retention mechanism of the HPG-Sil 3 phase， a set of model polar compounds， including thymine， uracil， hypoxanthine， and adenosine， was selected. The influence of critical mobile phase parameters on the analyte retention factor （k） was examined， including the acetonitrile （ACN） content， the concentration of ammonium acetate buffer， and the buffer pH. The observed retention trends were unequivocally indicative of a dominant HILIC mechanism. A pronounced increase in retention with increasing ACN content was observed for all analytes， consistent with the enhanced partitioning of solutes into a water-rich layer immobilized on the hydrophilic stationary phase surface. Conversely， an increase in buffer concentration led to a decrease in retention， a phenomenon explained by the competitive adsorption of buffer ions with the analytes for polar interaction sites on the HPG layer. Furthermore， the retention of ionizable analytes， namely hypoxanthine and adenosine， was demonstrably influenced by the buffer pH， as pH variations alter their ionization state and thus their overall hydrophilicity and interaction strength with the stationary phase. Under the optimized chromatographic conditions， all four model analytes were baseline separated within a 10-min runtime， exhibiting excellent peak symmetry and high repeatability. The practical utility of the HPG-Sil 3 stationary phase was further demonstrated through the efficient separation of alginate oligosaccharide homologs with degrees of polymerization （DP） ranging from 2 to 7. The elution order followed increasing DP， which aligns perfectly with the HILIC retention principle， as larger oligosaccharides possess more hydroxyl groups and exhibit stronger hydrophilic interactions. The stationary phase also demonstrated remarkable operational stability， with no observable changes in retention times or chromatographic efficiency over 10 consecutive injections， underscoring its robustness for routine analytical applications. In conclusion， the HPG-Sil 3 stationary phase synthesized in this work exhibits outstanding hydrophilic separation performance， a well-understood HILIC retention mechanism， and excellent long-term stability. These attributes position it as a highly competitive and promising material for the analysis of a wide array of polar compounds， including nucleosides， carbohydrates， and polar pharmaceuticals， across diverse fields such as metabolomics， biopharmaceutical analysis， and food chemistry. Future research will focus on the precise control of polymer parameters， including molecular weight， branching density， and post-functionalization， to fine-tune the properties of the HPG layer for tailored selectivity towards specific applications. Furthermore， a systematic evaluation of its performance using complex biological and environmental matrices will be conducted to assess its practical applicability.

Key words: hyperbranched polyglycerol, hydrophilic chromatographic stationary phase, hydrophilic mechanism, alginate oligosaccharides

CLC Number:

O658

JIAO Ruiwen, ZHANG Dandan, HU Xingyuan, ZHU Wanting, LI Xiang, CAI Yidi, REN Dandan, WANG Qiukuan, WU Long, ZHOU Hui. Preparation of a novel polyhydroxy silica stationary phase and application in the separation of alginate oligosaccharides[J]. Chinese Journal of Chromatography, 2026, 44(4): 373-382.

Figures/Tables 13

Fig. 1 Synthesis route of HPG-Sil

Fig. 2 FTIR spectra of SiO2 and HPG-Sil 3

Table 1 Elemental analysis results of SiO2 and silicon spheres grafted with different layers of HPG-Sil

Analyte	Elemental contents/%		Surface coverage/（μmol/m²）
Analyte	C	H	Surface coverage/（μmol/m²）
SiO₂	0.25	0.367	/
HPG-Sil 1	2.22	0.598	1.41
HPG-Sil 3	4.9	0.673	3.30

Fig. 3 TGA curves of SiO2 and HPG-Sil 3

Fig. 4 SEM images of （a） SiO2 and （b， c） HPG-Sil 3， and TEM images of （d） SiO2 and （e， f） HPG-Sil 3

Fig. 5 （a） N2 adsorption-desorption isotherms of SiO2 and HPG-Sil 3， and （b） pore distribution of bare silica and HPG-Sil 3

Table 2 Surface areas， pore volumes and pore diameters of SiO2 and HPG-Sil 3 stationary phase

Analyte	Specific surface area/（m²/g）	Pore volume/（cm³/g）	Pore diameter/nm
SiO₂	458.41	1.07	12.57
HPG-Sil 3	301.13	0.95	9.33

Fig. 6 Chromatograms of analytes using different stationary phase chromatographic columnsMobile phase： acetonitrile-water （85∶15， V/V）； flow rate： 0.6 mL/min； column temperature： 35 ℃； buffer salt concentration： 40 mmol/L； pH： 7.5； detection wavelength： 254 nm.Peak identifications： 1. thymine； 2. uracil； 3. hypoxanthine； 4. adenosine.

Fig. 7 Effect of acetonitrile volume fraction on the retention factors （k） of analytes on the HPG-Sil 3 stationary phases

Fig. 8 Effect of ammonium formate concentration in the mobile phase on k of the analytes on the HPG-Sil 3 stationary phase

Fig. 9 Effect of the mobile phase buffer pH on k of the analytes on the HPG-Sil 3 stationary phase

Fig. 10 Retention and distribution evaluation of HPG-Sil 3 stationary phase in hydrophilic mode mechanism investigationa. plot of distribution model； b. adsorption model； c. multi-factors retention model.

Fig. 11 Chromatogram of alginate oligosaccharides separated by HPG-Sil 3 stationary phase under optimized conditionsNumbers indicated the degree of polymerization.

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